By the word vacuum is meant the condition where the gas pressure is reduced to significantly less than atmospheric pressure. On the surface of the earth, such conditions can only be sustained in an enclosure that is impervious to gas flow and from which atmospheric gases are actively removed by some kind of vacuum pump. Gaseous pressure is measured officially in SI units of Pascals, or Newtons/m.sup.2, and more commonly in units of Torr, which relates to the pressure required to support a column of Mercury at a height of 1 millimeter. Mean atmospheric pressure is 1.013.times.10.sup.5 Pa or 760 Torr.
Industrial applications for vacuum (and hence vacuum pumps) include chemical procedures in which materials are fabricated or modified in a controlled environment. Thin film processing, by deposition, cleaning, and etching of material under vacuum forms one of the largest applications. More specifically, these processes are used in the manufacture of large-scale integrated electronic circuits where the goal of small feature size requires the cleanliness and the chemical control that is available in a vacuum.
Existing vacuum pumps fall into several classes according to the principle used to achieve a pumping action. Each class of pump is found to be effective over a characteristic pressure range. Often, two or more stages of pumping are utilized to achieve or maintain a pressure differential from high vacuum to atmospheric pressure: The initial stages compress the gas from a very low pressure to successively higher ones, and the final stage compresses the gas further for exhaust to atmosphere. Final-stage, or backing, pumps commonly are of a mechanical piston design. Oil-lubricated mechanical pumps can operate at inlet pressures down to 10.sup.-3 Torr; dry-lubricated pumps required for vacuum processes that cannot tolerate oil contamination go down to 10.sup.-2 Torr.
By the word plasma is meant the state of matter where atoms have been ionized, resulting in a gas of electrically charged ions and electrons. Vacuum pumps using a plasma as the working fluid have been proposed before: In particular, reference is made to U.S. Pat. No. 4,641,060 issued Feb. 3, 1987 to R. A. Dandl, where the gas being pumped is ionized and constrained to flow along magnetic field lines through baffle structures that restrict the backflow of neutral gases. The Dandl pump cannot compress the pumped gas to pressures higher than 10.sup.-3 Torr, which is a handicap for industrial processes requiring extreme cleanliness and oil-free backing pumps.
By the word magnetohydrodynamics and its abbreviation MHD is meant a fluid-model description of a plasma wherein the electrical resistivity of the plasma is not important. Under these conditions, the plasma behaves as a fluid whose electrical currents are influenced by magnetic fields. Specifically, the force equation for the fluid plasma is: ##EQU1## where .rho. is the mass density of the plasma, v is the velocity of a fluid element of the plasma, .gradient.p is the gradient of the plasma pressure, j is the plasma electrical current, and B is the magnetic field vector.
An object of the present invention is to be an effective initial or intermediate stage in a clean, oil-free vacuum pumping system, suitable for processing applications requiring stringent purity and chemical control. Specifically, this requires the achievement of a compression of the pumped gas to a minimum of 10.sup.-2 Torr so that oil-free backing pumps may be used.
A further object of this invention is to be a pump with high throughput when the inlet pressure is in the range of 10.sup.-4 to 10.sup.-3 Torr to maximize the processing rate in deposition, cleaning, and etching operations.
A further object of this invention is to be a device which destroys toxic or corrosive gases in the effluent which is simultaneously being pumped by this device.
A further object of this invention is to be a device in which the elevated temperatures of a plasma formed from pre-selected reactants promotes desired chemical reactions in thin-film surfaces onto which the plasma impinges.